a) Field of the Invention
The present invention relates to a field effect semiconductor device, and more particularly to a field effect compound semiconductor device suitable for operating at a large signal amplitude and at a high frequency.
b) Description of the Related Art
Compound semiconductors generally have a higher carrier mobility than silicon. Field effect semiconductor devices having a compound semiconductor channel layer are known as semiconductor devices capable of operating at a high frequency. For example, these field effect semiconductor devices are widely used in the fields of supercomputers, microwave communications, and the like. In particular, semiconductor devices used for mobile station microwave communications are required to have a small power consumption and high efficiency.
A metal-semiconductor field effect transistor (MESFET) uses compound semiconductor and has a Schottky electrode. Use of compound semiconductor enables MESFET to operate at a high speed.
A current flowing between the drain and source of a MESFET is controlled by applying a backward bias voltage to the Schottky gate electrode so as to develop a depletion layer. In a large signal operation, a signal having a large amplitude is applied to the gate electrode so that the voltage at the gate electrode changes greatly. When a large backward bias voltage is instantaneously applied to the gate electrode, the drain-source current reduces and the drain voltage rises. As a result, a high voltage is applied between the gate electrode and drain electrode. When the drain-gate voltage exceeds the breakdown voltage of the Schottky gate electrode, a backward gate current flows.
When a forward bias voltage is instantaneously applied to the gate electrode, a forward gate current flows from the gate electrode to the source electrode. This forward gate current substantially deepens the gate bias, thus reducing the drain current.
It is preferable that a Schottky gate electrode has a high backward breakdown voltage and that a forward gate current is easily injected during the forward bias operation.
In order to realize a MESFET with a Schottky gate electrode having a high backward breakdown voltage, it has been proposed to form a potential barrier layer with a wide band gap between the channel layer and Schottky electrode. Although this potential barrier layer improves tile backward breakdown voltage of the Schottky gate electrode, the current injection ability during the forward bias operation is lowered.